Mass spectrometry (MS) is widely used as an analytical technique to provide qualitative and quantitative analysis of sample components. Generally, sample components are converted into ions which are resolved according to their mass-to-charge ratios. The ions are collected at an ion detector which converts the mass-resolved ion signals into output electrical signals. Typically, the ion detector includes an electron multiplier stage that applies voltage and thus provides gain to the output electrical signal of the ion detector. The output electrical signals are then processed to produce a mass spectrum.
In mass spectrometry it desirable for the spectrometer to operate over a wide range so that ions having very low intensities and ions having high ion intensities can be measured in the same mass scan. The measure of such performance is characterized as the dynamic range of the ion detector or mass spectrometer, and is generally defined as the range of output electrical current values across which the electron multiplier will provide a linear response. A wide dynamic range is difficult to achieve however, because for one voltage setting of the ion detector gain, either the large ion signals become saturated or the very low ion signals are not detected. Thus, the user would traditionally have to manually adjust the detector or multiplier gain for the two extreme conditions.
U.S. Pat. Nos. 7,047,144 and 7,745,781, the disclosures of both of which are hereby incorporated by reference in their entirety, describe techniques to address this problem by monitoring the ion intensities as they are detected and changing the multiplier voltage and thus the applied gain so that ions of all intensities are detected. In some examples, when the received ion signal intensity is very high, the multiplier voltage is decreased, and the ion signal is multiplied by a pre-tuned compensation factor or gain in order to compensate for the decrease in the voltage multiplier. When the received ion signal intensity is too low, the multiplier voltage is increased and applied to the ion signal to adjust the ion intensity accordingly. With this method, both sides of the extremes in received ion signal intensities are compensated for, which increases the dynamic range of the ion detector (sometimes also referred to as “extended dynamic range” or “EDR”). Because of dynamic range, when we have high ion intensity, the multiplier voltage is reduced which in turn increases the compensation factor used to multiply the signal.
While the methods described in U.S. Pat. Nos. 7,047,144 and 7,745,781 are an advance in the art, a significant limitation of the prior art is that the method does not differentiate between the actual ion signal and the electronic baseline signal of the ion detector. Specifically, the same compensation factor is used to compute the height of all signals, both the ion signals and the electronic baseline signal. The electronic baseline signal is independent of the ion signals, and when extended dynamic range is applied to all of the signals in a spectrum, meaning that as all of the signals are multiplied up and down by a selected compensation factor due to the variations in large and small peak intensities of the ion signals, the baseline signal value is also multiplied up and down which may cause the baseline signal to appear as one or more false peaks when the output signals are processed. FIG. 1 depicts such a problem. In FIG. 1 a mass chromatograph produced by prior art methods of applying extended dynamic range techniques to the ion detector is illustrated. As shown, a real peak 102 is present, however since the baseline signal is also multiplied by the selected compensation factor, a number of false peaks 104, 106 and 108 are produced. Thus, when the output signals are processed and a chromatograph of different masses is produced you will still see peak(s) from the baseline signal, irrespective of whether the ion actually present in the sample or not.
False peaks in the resulting mass chromatograph are a significant problem for the industry. False peaks can be misinterpreted as real ion signals leading to misidentification of sample constituents and erroneous results. Such problems limit the use and effectiveness of techniques for improving sensitivity and extending the dynamic range of the instruments. Accordingly, additional developments and improvements are greatly needed.